SPM
Notes

Inventors. The first
scanning
probe microscope was the scanning tunneling microscope (STM) of Binnig
and Rohrer (Binnig, G., Rohrer, H., et al., (1982) Phys.
Rev.
Lett., 49:57.). Gerd Binnig and Heinrich Rohrer were awarded
half of the 1986
Nobel Laureate in Physics for their design of the
scanning
tunneling microscope. Ivan Amato's 1997 article, "Atomic
Imaging:
Candid Cameras for the Nanoworld " (Science276(5321):1982-1985),
entertainingly recounts the history of STM and AFM
development.
The article is available on-line to Science subscribers.
A
history
of microscopy (Microscopy for Nanotechnologists by C.
David
Eagle) provides perspective on the explosive development of this field.

Scanning. The probe (or
the
sample under a stationary probe) generally is moved by a piezoelectric
tube. Such scanners are designed to be moved precisely in
any
of the three perpendicular axes (x,y,z). By following a raster
pattern,
the sensor data forms an image of the probe-surface interaction.
Feedback from the sensor is used to maintain the probe at a constant
force
or distance from the object surface. For atomic force microscopy
the sensor is a position-sensitive photodetector that records the angle
of reflection from a laser bean focused on the top of the cantilever.

AFM systemsdetect
the
z-displacement of the cantilever by the reflection of a laser beam
focused
on the top surface of the cantilever. The feedback from this
sensor
maintains the probe at a constant force.

STM systems measure the
quantum
tunnelling current between a wire or metal-coated silicon tip and the
object
surface. An electronic feedback system maintains a constant
current
by positioning the tip to exactly contact the surface. [ More
detail ]

NSOM systems scan an optical fiber
probe over the sample. The probe has an opaque material covering its
surface,
except for a small aperture at the tip. The light (usually a
laser
source is used) is emitted through this aperture. Image data can be
gathered
in transmission, reflection or fluorescence mode. The
transmission
mode provides a higher signal throughput. It can be used with
specimens
that are transparent and have low or moderate light absorption,
particularly
biological subjects. Reflection mode is for highly scattering and
opaque samples. The resolution of optical microscopes has been
limited
by the wavelength of light, in practice about 400 - 500 nm. By placing
a point source of light less than that distance from the sample, NSOM
improves
this resolution by an order of magnitude. An NSOM is available
from
ThermoMicroscopes
.

Interaction force.
The z-axis (vertical) component of the force of interaction is
calculated
from the z-displacement of the cantilever and the spring
constantof the cantilever. From Hooke's Law, F = - kz,where
kis
the spring constant. The spring constantfor a
cantilever
is provided by the cantilever supplier or can
be determined by the investigator . A constant forceon the
probe
tip is maintained by feedback from measurement of the interaction
force.
The probe is moved up and down to maintain the measured constant force.

Tapping
modeTM, a trademark of Digital
Instruments . Tapping Mode imaging is implemented in ambient
air by oscillating the cantilever assembly at or near the cantilever’s
resonant frequency using a piezoelectric crystal.. To image
in fluids, the entire fluid cell is oscillated to drive the cantilever
into oscillation.

Tip
Selection.. AFM tips are generally made of silicon
or silicon nitride. For most applications, pyramidal
silicon nitride tips are used. They are relatively durable
and
present a hydrophobic surface to the sample. Conical
silicon tips are often used for bio-molecular applications because
they are very sharp and present a hydrophilic surface. However,
they
are relatively less durable. For the ultimate sharpness, tips of
carbon nanotubes have
been
made. The Rice group also has a tutorial
for mounting carbon nanotube tips on commercial cantilevers.
In other cases selective modification of silicon nitride tips has been
used to provide for measurement of specific molecular
interactions.
STM tips are made of mechanically-formed or electrochemically-etched
wire,
usually noble metals or tungsten. Digital Instruments has a
useful
Tip
Selection Guide . Tips are available from
many
suppliers.

Several web sites offer information and/or references of use to
those
desiring to construct these instruments. See for example, the
site
of the J.C.
Davis
group, Cornell University. Construction
of the tip is, perhaps, the most crucial aspect of the experiment.

Mervyn Miles published a useful overview of AFM technology
and
applications,
Scanning Probe Microscopy: Probing the Future
(1997, Science277(5333):1845).
The article is available on-line to Science subscribers.

Sample preparation is of great
importance
in SPM as in other areas of microscopy. SPM analysis of
biological
macromolecules places particularly high demands on the quality of the
substrate.
Freshly-cleaved mica surfaces has been particularly useful. Mica
presents a charged, hydrophilic surface to which proteins and other
biomolecules
readily bind. Moreover, mica surfaces are nearly flat on an
atomic
scale and are quite clean when fresh, conditions that are ideal for
scanning
at high resolution. For certain applications, covalent
attachment
to the surface is be required. A particularly useful approach has
been the preparation of gold surfaces coated with protein-reactive
monolayers.
Several investigators have used monolayers composed of alkanethiols and
dithioalkanes. Another variation uses N-hydroxysuccinimide ester
functionalized
monolayers on a gold surface. Digital Instruments has a large
collection
of application
notes online.

Digital Instruments provides a variety of technical and
scientific
resources in their Library
. They also sponsor an e-mail discussion group, The
SPM Forum/Mailing List, "an open and unedited, but non-commercial,
forum for discussing and exchanging technical information, views,
issues,
and applications of SPM." "Anyone actively involved in SPM usage is
invited to join the Digest." [ follow
link to subscribe ].